Mesoporous titanium dioxide nanoparticles and process for their production

ABSTRACT

TiO 2  nanoparticles having improved consistent particle morphology, uniform particle size, and which contain uniform intra-particle pores in the mesopore size range are produced by wet chemical hydrolysis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/297,869, filed Nov. 16, 2011, the entirety of which is herebyexpressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

The presently disclosed and claimed inventive concept(s) relategenerally to mesoporous titanium dioxide (TiO₂) nanoparticles, and moreparticularly, to a new type of TiO₂ nanoparticles which are highlyuniform in particle size and contain generally uniform intra-particlepores in the mesopore size range.

Porous nanoparticles, especially those with well defined pores and anarrow pore size distribution, have high application potentials incatalysis, as catalyst supports, adsorbents, in optics, photovoltaics,and filtration materials for separation. Control of particlemicrostructure allows control of physical and electronic properties,which, in turn, leads to new functionalized materials.

U.S. Patent Application Pub. Nos. 2006/0110316 and 2011/0171533 relate,respectively, to mesoporoous metal oxides and to a mesoporous oxide oftitanium which can be produced by precipitating an ionic porogen and ahydrous oxide of the metal, i.e., titanium, compound comprising atitanium starting material, a base and a solvent, wherein the titaniumstarting material or the solvent or both are a source of the anion forthe ionic porogen and the base is the source of the cation for the ionicporogen. The ionic progen is removed from the precipitate, and themesoporous oxide of titanium is recovered. However, there is a need fora process for preparing TiO₂ nanoparticles that demonstrate consistentparticle morphology, uniform particle size, spherical shape, and whichcontain uniform intra-particle pores in the mesopore size range.

SUMMARY OF THE INVENTION

The presently disclosed and claimed inventive concept(s) relates to anoxide of titanium, i.e., TiO₂, in the form of a generally uniformspherical nanoparticle of from about 20 nm to about 100 nm in sizewherein each particle comprises generally uniform intra-particlemesopores having a substantially uniform pore size distribution centeredat a value between about 2 nm to about 12 nm. In a preferred embodiment,the TiO₂ nanoparticles are generally spherical in the range of 50 nm insize and exhibit intra-particle mesopores centered at about 6 nm.

The TiO₂ nanoparticles are a powder material wherein the nanoparticlesexhibit a bimodal pore size distribution. One mode is from theintra-particle pores mentioned above, i.e., the pores within individualnanoparticles. The other mode originates from the packing arrangement ofthe nanoparticles, that is, the inter-spacial pores, with asubstantially uniform pore size distribution centered between about 15nm to about 80 nm. In a preferred embodiment, the TiO₂ powder materialformed by this type of nanoparticles has a substantially uniforminter-particle pore size distribution centered at about 35 nm.

The TiO₂ nanoparticles according to the presently disclosed and claimedinventive concept(s) are produced by:

-   (i) forming an aqueous solution of a water soluble compound of    titanium at a concentration of from 0.5 to 1.5 moles per liter in    the presence of an organic mineral acid at an acid to titanium molar    ratio of from 0.02 to 0.2;-   (ii) heating the aqueous solution to a temperature in the range of    from 70° C. to 80° C. and maintaining that temperature for a period    of from 1 hour to 3 hours, and then heating the aqueous solution to    a temperature in the range of from 100° C. up to the refluxing    temperature and maintaining that temperature for an additional    period of from 2 hours to 4 hours;-   (iii) cooling the solution to room or ambient temperature, i.e., a    temperature in the range of 25° C., and separating the reaction    product.

The process of the invention is capable of producing a consistentlyuniform particle size for this type of TiO₂ nanoparticle that can becontrolled at a size range of from about 20 nm to about 100 nm. Theintra-particle mesopores exhibit a narrow pore size distributioncentered at a value between about 2 nm to about 12 nm. The powdermaterial from this type of nanoparticles also exhibit substantiallyuniform inter-particle pores with a pore size distribution centeredbetween about 15 nm to about 80 nm. Pore size distribution measurementsvia N₂ adsorption (BET) on one of this type of nanoparticle productsreveal that the material has two types of mesopores. One type ofmesopore, i.e., the intra-particle mesopore, is centered at about 6 nm,which has also been observed by SEM. The other type of pores is centeredat about 35 nm, and these are believed to be inter-particle poresgenerated by the packing arrangement of the individual nanoparticles.Nanoparticle materials produced according to the described and claimedinventive concept(s) have a pore volume of from 0.2 to 0.6 cm³/g asmeasured by N₂ adsorption (BJH).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B depict scanning electron microscope (SEM) images of SEMimages of mesoporous TiO₂ nanoparticle samples produced according to theinventive concepts described herein.

FIG. 2 is a pore size distribution plot of the TiO₂ nanoparticlesproduced according to the inventive concepts described in Example 1.

FIG. 3 is a pore size distribution plot of the TiO₂ nanoparticlesproduced according to the inventive concepts described in Example 2 withone sample treated at a temperature of 200° C. and a second sampletreated at a temperature of 300° C.

FIG. 4 is a pore size distribution plot of the TiO₂ nanoparticlesproduced according to the inventive concepts described in Example 3.

FIG. 5 is a pore size distribution plot of the TiO₂ nanoparticlesproduced according to the inventive concepts described in Example 4.

DETAILED DESCRIPTION OF THE INVENTION

The disclosed and claimed inventive concept(s) are directed to a processfor producing a type of TiO₂ nanoparticles which are highly uniform inparticle size and contain uniform intra-particle pores in the mesoporesize range with a relatively narrow pore size distribution. As usedherein, the term “mesoporous” or “mesopore size range” means structureshaving an average pore diameter of from 2 nm to 100 nm (20 Å up to 1000Å), although the average pore diameters of the structures are generallysmaller than 100 nm depending on the morphology of the nanoparticles,which in turn depends on the titanium precursor material, i.e., thewater soluble compound of titanium, and the entity of the organic acidsused in the process.

The highly uniform and generally spherical TiO₂ nanoparticles of theinvention are produced by:

-   (i) forming an aqueous solution of a water soluble compound of    titanium at a concentration of from 0.5 to 1.5 moles per liter in    the presence of an organic acid at an acid to titanium molar ratio    of from 0.02 to 0.2;-   (ii) heating the aqueous solution to a temperature in the range of    from 70° C. to 80° C. and maintaining that temperature for a period    of from 1 hour to 3 hours, and then heating the aqueous solution to    a temperature in the range of from 100° C. up to the refluxing    temperature and maintaining that temperature for an additional    period of from 2 hours to 4 hours;-   (iii) cooling the solution to room or ambient temperature, and    separating the reaction product.

The reaction product is typically separated as a powder, and the powderis then treated to remove solvent (e.g., water) from the pores, such as,for example, by heating the powder at a controlled temperature in therange of from 200° C. to 500° C.

As noted above, preparation of mesopopous TiO₂ nanoparticles accordingto the described and claimed inventive concept(s) begins with preparingprecursor nanoparticles by wet chemical hydrolysis. A typical hydrolysisprocess involves the following steps:

Dissolving a water soluble compound of titanium in distilled ordeionized water at a titanium concentration of 0.5 to 1.5 moles perliter. Optionally, a small amount of an inorganic acid can be added tocontrol the solution pH and function as a hydrolysis catalyst to speedup the hydrolytic reaction. A suitable amount of an organic acid is thenadded to the reaction mixture typically at an acid to titanium molarratio of 0.02 to 0.2. The organic acid has been observed to function asa morphology controlling agent.

The solution, i.e., reaction mixture, thus formed is transferred to aheated reactor equipped with a condenser, and the solution is heated toa temperature between about 70° C. and 80° C. As an option, anatase TiO₂seeds can be added to the solution at a seed to TiO₂ molar ratio of from0.0005 to 0.0015 while maintaining the same solution temperature for aperiod of from 1 hour to 3 hours. The TiO₂ seeds operate to control thecrystalline phase and particle size of the nanoparticles. Thereafter thereactor temperature is raised to a value in the range of from 100° C. upto the refluxing temperature and maintained at that temperature for anadditional period of from 2 hours to 4 hours.

The reaction is then cooled to room or ambient temperature, and thereaction product can be separated by filtration and then washed withdeionized water until it is substantially free of the salts generatedduring hydrolysis. The reaction mixture can also be neutralized with abase, such as, for example, an ammonia solution, a sodium hydroxidesolution, and the like, before filtration and washing.

The precursor nanoparticles thus formed are then treated to removeadsorbed water and residue acid molecules from their pores to producethe mesoporous nanoparticles of the invention. This treatment can beaccomplished in a number of different ways known to those skilled in theart. For example, common organic solvents which are miscible with water,such as ethanol, propanol, acetone, tetrahydroforan, and the like, maybe used to extract water from the precursor nanoparticles. Lowtemperature drying, e.g., in the temperature range of from 60° C. to150° C., may be necessary to remove the solvents after extraction.Strong desiccants may also be used to remove adsorbed water from thenanoparticles. For example, phosphorus pentoxide or concentratedsulfuric acid may be used to dry the nanoparticles in a desiccator withthe sample sitting over the desiccants. A few days may be required forthe adsorbed water to be completely removed from the mesopores. Incertain cases, when detected, organic acid residue, such as citratemolecules, may need to be removed to further free up the pores. Organicacid residue may be removed by washing the nanoparticles with a saltsolution, such as ammonium bicarbonate, after they have been washed withdeionized water. A simple, effective and preferred method for removingadsorbed water and a majority of the residue acid molecules is byheating the nanoparticles in an oven at a temperature of from 200° C. to500° C. under a constant air flow.

For the precursor materials for preparing the TiO₂ nanoparticlesaccording to the inventive concept(s) described and claimed herein, anywater soluble compounds of titanium may be used in the thermalhydrolysis. These include, but are not limited to, titanium oxychloride,titanium oxysulfate, and the like; titanium potassium oxalate and thelike; titanium bis(ammonium lactate) dihydroxide, bis-acetylacetonetitanate and other water soluble titanium complexes. Suitable organicacids for use in the process are alpha hydroxyl carboxylic acids andinclude citric acid, tartaric acid, malic acid and the like. Citric acidis preferred in cases where nanoparticles having a spherical shape aredesired.

The SEM images shown in FIGS. 1A and 1B depict spherical mesoporous TiO₂nanoparticles produced according to the inventive concepts describedherein. The TiO₂ particles are highly uniform in particle size, with thesamples shown in FIGS. 1A and 1B having a particle size of about 50 nm.The intra-particle pores are about a few nanometers in size, and theycan be clearly seen under SEM. BET measurements indicate that thesamples exhibit a bimodal pore size distribution with one type of porescentered at about 6 nm, which are the same intra-particle pores observedunder SEM. The nanoparticles exhibit a narrow pore size distributionwhich can be centered, for example, at 6 nm, 10 nm, 12 nm, etc. Theother type of pores observed are centered at about 35 nm, which isbelieved to be pores that are formed by the packing arrangement of theindividual 50 nm nanoparticles. Both types of the pores are in themesopore size range.

Example 1

1,196 g deionized water, 79 g hydrochloric acid solution (37% fromFisher Scientific), 5.9 g citric acid monohydrate (from Alfa Aesar) and398 g titanium oxychloride solution (25.1% in TiO₂, from MillenniumInorganic Chemicals) were mixed together in a heated reactor equippedwith a glass condenser and an overhead stirrer. While being constantlystirred, the mixture was heated to 75° C. and a small amount of anataseTiO₂ seeds (0.1% vs. TiO₂; anatase seeds were produced by MillenniumInorganic Chemicals) was quickly introduced. The reaction was maintainedat 75° C. for 2 hours. During this period, TiO₂ particles began to formthrough hydrolysis of the titanium oxychloride. The reaction temperaturewas then increased to 103° C., and the reaction mixture was maintainedfor 3 hours at that temperature. The hydrolysis was essentially completeat this stage.

The reaction mixture was then cooled to room temperature and transferredto a different container where the particles formed during the reactionwere allowed to settle for a few hours. After substantially all of theparticles had settled to the bottom of the container, the mother liquorwas removed and about the same amount of deionized water was added. Themixture was stirred to re-slurry the particles, and then the pH of theslurry was increased to about 7 by slow addition of an ammonia solution(˜29%, Fisher Scientific). The particles were then separated from theliquid using a Buchner filter and washed with deionized water until theconductivity of the filtrate was lowered to about 5 mS/cm. Then thefilter was filled with an ammonia bicarbonate solution having a solutionconductivity of about 5 mS/cm. The ammonia bicarbonate washed materialwas then heated in an oven at 300° C. for 6 hours under a flow of air.SEM measurement of the material showed that the particles prepared bythis process were spherical in shape and had an average particle size ofabout 50 nm. Each particle exhibited intra-particle mesopores of about afew nanometers in size (FIGS. 1A and 1B). BET measurement results showedthat the material had a surface area of 121 m²/g and a pore volume of0.6 cm³/g. X-ray diffraction (XRD) measurement showed an anatasecrystallite size of 11.9 nm. The pore size distribution plot shown inFIG. 2 indicates that the material exhibited a substantially bimodalpore size distribution centered at about 6 nm and 35 nm, respectively.

Example 2

TiO₂ nanoparticle samples were produced as described above in Example 1except that 0.15% (vs. TiO₂) anatase seeds were used instead of 0.1%.One half of the reaction product was degassed at 200° C. for ˜12 hoursbefore BET measurement; the other half of the reaction product wastreated in an oven at 300° C. for 6 hours. The pore size distributioncurves of the two samples measured by BET are shown in FIG. 3. As onecan see, both samples exhibit substantially bimodal pore sizedistribution. However, the sample degassed at 200° C. for 12 hours has asmaller pore size modal centered at 2.7 nm, while for the sample treatedin an oven at 300° C. for 6 hours, the pore size distribution modalincreased to 5.4 nm. The larger pore size modal is also increased by afew nanometers. Other measurement data indicated that the 200° C. sampleexhibited a surface area 308 m²/g, a pore volume of 0.52 cm³/g, and acrystallite size (XRD) of 6.6 nm for the sample before thermal treatmentin an oven. For the 300° C. sample, surface area was 128 m²/g, porevolume was 0.46 cm³/g and crystallite size (XRD) was 11.3 nm beforethermal treatment in an oven.

Example 3

A TiO₂ nanoparticle sample was produced as described above in Example 1except that anatase seeds were not used. The product was degassed at200° C. for about 12 hours before BET measurement. A plot of the poresize distribution for the sample, measured by BET, is shown in FIG. 4.The plot also shows a bimodal pore size distribution with the smallerpore size modal being centered at 4.7 nm and the larger pore size modalbeing centered at 22 nm. The sample had a surface area of 218 m²/g, apore volume of 0.2 cm³/g, and a crystallite size (XRD) of 6.3 nm.

Example 4

TiO₂ nanoparticle samples were produced as described above Example 1except that no seeds were used, and 6.6 g of citric acid monohydrate wasadded instead of 5.9 g. One third of the reaction product was degassedat 200° C. for about 12 hours before BET measurement. Another third ofthe reaction product was treated in an oven at 300° C. for 6 hours, andthe last third of the reaction product was treated in an oven at 500° C.for 6 hours. Pore size distribution plots for the three samples measuredby BET are shown in FIG. 5. All three samples exhibit substantiallybimodal pore size distribution. However, the sample that was degassed at200° C. exhibits a pore size modal centered at 2.2 nm, whereas thesample treated in an oven at 300° C. exhibits a somewhat larger poresize modal centered at 5.4 nm. The sample treated in an oven at 500° C.exhibits an even larger pore size modal centered at 8.6 nm. Surface areafor the 200° C. sample was measured at 262 m²/g, and pore volume wasmeasured at 0.28 cm³/g. Surface area for the 300° C. sample was measuredat 115 m²/g, and pore volume was measured at 0.32 cm³/g. Surface areafor the 500° C. sample was measured at 58 m²/g, and pore volume wasmeasured at 0.27 cm³/g. Crystallite size (XRD) was 6.4 nm for the firstsample before thermal treatment, 11.2 nm for the 300° C. sample, and19.2 nm for the 500° C. sample.

TiO₂ nanoparticles produced according to the invention demonstrateimproved consistent particle morphology, uniform particle size, andcontain substantially uniform intra-particle pores in the mesopore sizerange.

What is claimed is:
 1. A method for producing generally uniform TiO₂nanoparticles of from 20 nm to 100 nm in size wherein each particlecontains generally uniform pores in the mesopore size range having agenerally narrow pore size distribution centered at a value betweenabout 2 nm to about 12 nm comprising: (i) forming an aqueous solution ofa water soluble compound of titanium at a concentration of from 0.5 to1.5 moles per liter in the presence of an alpha hydroxyl carboxylic acidat an acid to titanium molar ratio of from 0.02 to 0.2; (ii) heating theaqueous solution to a temperature in the range of from 70° C. to 80° C.and maintaining that temperature for a period of from 1 hour to 3 hours,and then heating the aqueous solution to a temperature in the range offrom 100° C. up to the refluxing temperature and maintaining thattemperature for an additional period of from 2 hours to 4 hours; (iii)cooling the solution to room or ambient temperature, and separating thereaction product.
 2. The method of claim 1 wherein the reaction productis separated by (i) filtering; (ii) washing the separated reactionproduct to remove salts generated during the reaction sequence; and(iii) finishing the product by drying whereby water and organic contentsare removed.
 3. Method of claim 2 wherein finishing is achieved byheating the product at an elevated temperature in the range of from 200°C. to 500° C. under a flow of air.
 4. The method of claim 1, claim 2 orclaim 3 wherein heating the aqueous solution to a temperature in therange of from 70° C. to 80° C. and maintaining that temperature for aperiod of from 1 hour to 3 hours according to step (ii) is conductedconcurrently with adding anatase TiO₂ seeds to the aqueous solution at aseed to TiO₂ molar ratio of from 0.0005 to 0.0015.